Various circuitry for enabling and supporting the use of relatively high frequency signals has been utilized for a number of years. For a number of reasons, such as the availability of relatively unused spectrum, radiation providing penetration of a wide variety of materials, etc., the use of signals at higher and higher frequencies has become of interest. For example, the terahertz (THz) band from 0.3 THz to 3 THz is gaining increasing interest due to its potential for use with respect to various applications, such as imaging, spectroscopy, and high-speed wireless communication.
Unfortunately, however, a technology vacuum known as “terahertz gap” exists with respect to practical and satisfactory circuit implementations for enabling and supporting the use of signals in the terahertz band. For example, major challenges are presented with respect to implementing radiating sources operable at terahertz frequencies with sufficient output power to overcome the severe path loss at such high frequencies. Currently, most terahertz sources are based on optics, quantum cascade lasers, and Gunn diodes, which are bulky and expensive making implementations of such existing circuitry undesirable and even unacceptable for many applications, such as mobile communications devices, mass-produced consumer devices, and battery powered devices.
Although some recent works have demonstrated sources and radiators from sub-THz to THz in bulk complementary metal-oxide-semiconductor (CMOS) based circuits, the inadequate cut-off frequency (fT) and the low breakdown voltage of CMOS transistors significantly limit the direct current (DC) to radio frequency (RF) (DC-to-RF) conversion efficiency. In particular, many existing circuit implementations rely on relatively high-order harmonics (e.g., 4th order harmonics) for frequency generation above 300 GHz due to insufficient fundamental oscillation frequency for lower-order harmonic extraction. As a result, the power efficiency of these existing circuits is typically as low as 0.05% and thus providing required output power levels is problematic without unacceptable power consumption and/or causing device breakdown. Phase noise, in addition to the aforementioned output power, is also an important requirement for terahertz sources utilized in many applications, such as imaging and time-domain spectroscopy. The phase noise realized by existing circuits in operation around 300 GHz is typically worse than −85 dBc/Hz at 1 MHz offset (e.g., −78 to −85 dBc/Hz at 1 MHz offset). Accordingly, the phase noise and the DC-to-RF efficiency of existing CMOS based terahertz circuit implementations are quite limited.